The present invention is directed to the problem of addressing large-area liquid-crystal displays that do not require the costly processing steps: needed to incorporate a thin-film transistor, or some other switching device, into each subpixel.
The liquid-crystal displays currently used on most portable computers are classifiable as active-matrix LCDs. An active, i.e. switching, device incorporated into each subpixel of these displays transfers and holds a specified charge on a transparent electrode. These switches are typically field-effect transistors formed in thin films of amorphous silicon, or sometimes polycrystalline silicon. However, the capital investment required for the equipment needed to deposit silicon and form thin-film transistors (TFTs) with the necessary electronic switching behavior is high. Furthermore, this large investment must be repeated for each new generation of processing equipment capable of handling larger glass substrates. It appears now that the next (fourth) generation TFT-LCD fabs will use 800-mm by 950-mm substrates. The capital cost of the processing equipment per substrate divided by the number of 17.x-inch displays in the array (6), taking the yield of individual displays into account, should be attractive for desktop computing in the next few years.
Making large TFT-LCD backplanes one at a time in a 4th-generation fab would be inappropriate for a cost-sensitive application like consumer television. In the first place, the capital expense would be concentrated 6-fold compared to the 17.x-inch desktop display scenario. Furthermore, the yield for such large backplanes would be lower, raising the cost even higher. Finally, the maximum size at the desirable 16 by 9 aspect ratio would only be about 42 inches in diagonal, which is of marginal interest for standard definition (480-line) digital TV and clearly too small for high-definition (720-line or better) TV in the US market. In order to make LCDs at larger sizes and lower costs, it seems to be necessary to forego the advantages associated with creating an active element to control each sub-pixel.
On several occasions over the past quarter of a century, LCD technologists overcame what turned out to be only apparent limits to the information content that can be displayed without using active switching elements. FIG. 1 illustrates the first approach to this problem, which was described by Allan R. Kmetz in xe2x80x9cLiquid-Crystal Display Prospects in Perspective,xe2x80x9d IEEE Transactions on Electron Devices, Vol. ED-20 (1973) pp. 954-961. The pixels in a row are addressed together by applying a xe2x80x9cselectxe2x80x9d pulse to one row at a time while xe2x80x9cdataxe2x80x9d voltages are presented to the columns. All other (i.e. unselected) rows are kept at ground potential. The display is scanned, which means repeating this process for each row until all: the rows are addressed in what is called a frame. Therefore, addressing voltage waveforms must be repeated at some acceptable frame frequency. This frame rate is typically chosen to be at least 30 Hz so that the illusion of continuous motion can be created as the pixel patterns change from one frame to the next. However, it should be noted that the response time of the LCD may be different from the frame time.
The voltage applied to a pixel in FIG. 1 is equal to the difference between the select and data-voltage waveforms on the two electrodes defining that pixel. A larger voltage appears across the pixel if the row and column voltages have opposite sign. Now, it was known experimentally that nematic liquid crystals respond to the RMS (root mean-square or AC average) of the voltage applied to them. Choosing the data waveform to be xc2x1V0 makes the undesirable contribution to the RMS voltage (applied when other rows are being addressed) independent of the image. Furthermore, choosing, the select-pulse voltage VS to be exactly twice the magnitude, of the data voltage causes the voltage applied to an off pixel while it is being addressed to have this same value also. Thus the RMS voltage applied to an off pixel is V0 independent of the rest of the image. The voltage applied to turn a pixel on is 3V0 while the pixel is being addressed and xc2x1V0 the rest of the time. If there are N rows to be addressed, the ratio of the RMS voltages VON/VOFF turns out to be                                           V            ON                                V            OFF                          =                                            (                              1                +                                  8                  N                                            )                                      1              /              2                                .                                    (        1        )            
This ratio is not very large for N=240, which is appropriate for VGA or SDTV in the dual-scan configuration where the columns are split in the middle and independently driven from the top and the bottom. Although the rows are also independently driven, the two halves of a dual-scan display can be synchronized if necessary. Now the value of VON/VOFF that will be required to address a display depends on the details of the liquid-crystal composition and alignment. Turning the relationship around, we can see that this 3:1 method of addressing limits the number of addressable rows to:                               N          =                      8                                                            (                                                            V                      ON                                                              V                      OFF                                                        )                                2                            -              1                                      ,                            (        2        )            
when the on/off ratio is specified. For example, if the minimum ratio available is 1.2, only about 18 rows can be addressed. A dual-scanned display could have twice as many rows, but the total would still be discouragingly low for general purposes. For the more interesting case of NMAX=240, which allows a VGA or SDTV format with dual scanning, the on/off voltage ratio available is about 1.0165. This ratio was inadequate to switch known liquid-crystal configurations.
The 3:1 addressing scheme was predicated on the assumption that the RMS values of the unselected and off voltages should be the same. Whatever benefits that might have, it is not the same thing as maximizing the on/off ratio for a given number of rows. Paul M. Alt and Peter Pleshko showed this in xe2x80x9cScanning Limitations of Liquid-Crystal Displays,xe2x80x9d which appeared in IEEE Transactions on Electron Devices, Vol. ED-21 (1974) on pp. 146-155. In fact, a bigger on/off ratio can be obtained by increasing the select voltage relative to the data voltage as shown by the dashed pulse waveform in FIG. 1. The optimum ratio according to Alt and Pleshko is given by:                                           V            S                                V            0                          =                              N                          1              /              2                                .                                    (        3        )            
On the other hand, for a given value of the ratio of the RMS values of the on and off voltages, the maximum number of rows that can be addressed is given by:                               N          MAX                =                                            (                                                                                          (                                                                        V                          ON                                                                          V                          OFF                                                                    )                                        2                                    +                  1                                                                                            (                                                                        V                          ON                                                                          V                          OFF                                                                    )                                        2                                    -                  1                                            )                        2                    .                                    (        4        )            
For the interesting case of N=240, the on/off ratio that is needed turns out to be about 1.067. That was a big improvement over 3:1 addressing, but to be generally useful, a liquid-crystal configuration that could be switched by such a small ratio was still needed.
In 1985, Scheffer et al. reported their work on a liquid-crystal configuration that does respond to a reduced on/off voltage ratio in xe2x80x9c24xc3x9780 Character LCD Panel Using the Supertwisted Birefringence Effect,xe2x80x9d 1985 SID Symposium Digest of Papers, Vol. 16, pp. 120-123. This was the beginning of a new generation of what are now called supertwist or STN displays because the nematic liquid crystal twists by more than 90 degrees between the front and back substrates. STN displays with 240:1 multiplexing can be made to switch within the narrow voltage range available in Alt-Pleshko addressing. Dual-scan VGA STN displays have been used successfully for notebook computers but they were originally not practical at video rates. Of course, faster response can be obtained in active-matrix LCDs, where a thin-film switch is provided to control each subpixel. Unfortunately, AMLCDs are prohibitively expensive in the large sizes desirable for family entertainment. Furthermore, they have motion artifacts caused by the memory of the pixels between successive address times. This is because the human visual system tries to track an element moving quickly across a display, and the excessive persistence of the pixels smears the perceived image.
STN LCDs can be made with a fast-responding liquid-crystal material, but pixels relax between successive selections of the rows defining them when conventional Alt-Pleshko waveforms are used. This relaxation, which is called xe2x80x9cframe response,xe2x80x9d can be substantially complete in a typical frame time of {fraction (1/60)} sec. FIG. 2 illustrates pixel voltage and transmission waveforms for such extreme frame response. Y. Kaneko et al., first discussed frame response in a paper on xe2x80x9cFull-Color STN Video LCDs,xe2x80x9d which appeared in the Proceedings of Eurodisplay ""90, pp. 100-103. Up to that time, slow-responding liquid-crystal materials were often used, and STNs were sometimes erroneously thought to be inherently slow. However, Kaneko, et. al also showed that a fast-responding STN can be driven to its high-transmission state much faster than it relaxes back to low transmission in response to internal forces.
U.S. Pat. No. 5,420,604 xe2x80x9cLCD Addressing System,xe2x80x9d by Terry J. Scheffer and Benjamin R. Clifton, and issued on May 30, 1995, provides a method for addressing high-information-content LCD panels in which a fast-responding liquid-crystal material can be used. In this method, shown conceptually in FIG. 3, the row electrodes 0,1, . . . ,Nxe2x88x921 of the matrix are continuously driven by a set of orthogonal voltage waveforms F1, F2, . . . FN having a common period T. One way to construct orthogonal functions is to use harmonics of fundamental sine and cosine functions that have period T. Generally, the maximum frequency required to construct a set of N orthogonal functions is proportional to N. A particular column-voltage waveform Gj is formed by weighting the row waveforms with the pixel values corresponding to each row in that column. This method, which has been termed xe2x80x9cActive Addressing,xe2x80x9d provides the same on/off voltage ratio as Alt-Pleshko addressing, but the differential voltage delivered to an on pixel is spread throughout the frame time. Although this method substantially eliminates frame response, the viewing-angle range remains narrow at 240:1 multiplexing. Consequently, Active Addressing is beneficial for notebook computer displays, but it does not allow STNs to be made that are suitable for family entertainment.
K. F. Kongslie, R. G. Culter and P. J. Bos proposed another way to reduce frame response in their paper on xe2x80x9cA Synchronously Strobed Backlight for Improved Video-Rate STN Performance,xe2x80x9d 1994 SID Symposium Digest, Vol. 25, pp. 155-158. As shown in FIG. 4, the backlight consisted of a scannable array of 16 tubular fluorescent lamps. The lamps were pulsed one at a time, with variable delay, as the Alt-Pleshko addressing waveforms scanned through the adjacent rows of the display. A modest improvement in contrast was reported at the optimal delay, but it was not high enough for a video display. Furthermore, the experimental display required 75 ms to go from 10% transmission to 90% and vice versa, which is not fast enough for video. Kongslie et. al also reported a cross-talk problem in the vertical direction. This was most likely a parallax effect caused by viewing a row of pixels illuminated by the wrong lamp. Such pixels would show an exaggerated frame response caused by the increased delay between the times of addressing and illumination.
Some other work has been done with segmented backlights. For example, Roger G. Stewart and William R. Roach disclosed a xe2x80x9cField-sequential Display System Utilizing a Backlit LCD Pixel Array and Method For Forming an Imagexe2x80x9d in U.S. Pat. No. 5,337,068, which was issued on Aug. 9, 1994. In this work, the rows are addressed sequentially, but the problems encountered by Kongslie et. al may be avoided by the use of an active-matrix LCD. Unfortunately, however, a large-area active-matrix LCD would be prohibitively expensive for family entertainment. In U.S. Pat. No. 5,592,193, which issued on Jan. 7, 1997, Hsing-Yao Chen described a xe2x80x9cBacklighting arrangement for LCD Display Panel.xe2x80x9d Here, generally linear arrays of passive liquid-crystal elements are scanned in a sequential manner and illuminated by a partitioned backlight. A plurality of liquid-crystal video elements are activated at a time, but no specific method is given to accomplish this by sequential scanning. A coarsely partitioned backlight would illuminate elements that were scanned at widely varying times, so frame response could reappear to produce a non-uniform appearance within each plurality of elements. On the other hand, a backlight partitioned finely enough to avoid this reappearance of frame response could be costly. Furthermore, no provision was made to eliminate vertical parallax.
Thus there is still no LCD device that is practical at video rates and in the large sizes that are desirable for family entertainment. It is accordingly one object of the present invention to employ a novel driving arrangement in order to achieve uniformity with a scanning backlight and a fast STN display. Another object is to provide a lower effective multiplex ratio in order to obtain better contrast and a wider horizontal viewing-angle range. Yet another object of the invention is to reduce the visual impression of smearing of moving images by reducing the effective persistence of the pixels.
The display system of the present invention is directed to overcoming image-quality problems that arise according to the prior art when a scanning backlight illuminates a passive-matrix LCD containing a fast-responding liquid-crystal material.
Fast-responding STN LCDs generally turn on significantly faster than they turn off, but it has been difficult to exploit this fact using conventional addressing. An illuminated field of many rows would also show substantial frame response, while it is not practical at present to provide very finely segmented backlights. The present invention is directed to using a technique known as Active Addressing in fields to eliminate this frame response with a relatively coarsely-segmented scanning backlight.
The inventive system has a backlight that is divided into multiple independently addressable segments. A frame store accepts digital images at an input and forwards sub-images for a set of contiguous rows to a field store. A field contains rows that are illuminated by one of the segments. A complete frame is displayed by sequentially displaying all of its fields, and frames are updated at the rate defined at the input, which is commonly 60 Hz.
The controller also synchronizes a row-signal generator and a column-signal generator causing them to output signals to drivers that drive a field in the LCD by the method known as Active Addressing. More specifically, the row-signal generator generates a set of orthogonal waveforms and outputs them to row drivers, which are connected to row conductors on the display through switches. The orthogonal waveforms are independent of the image to be displayed. A column-signal generator receives the orthogonal waveforms from the row-signal generator and adds them together with weights proportional to the pixel values in the associated rows. A column signal is computed separately for each column in the display and sent to column drivers, which are attached to the column conductors in the display.
The period of the orthogonal waveforms is chosen so that each field is addressed for at least two periods. In the second period, after the pixels in a particular subfield have had enough time to turn on, the associated backlight segment illuminates the field thereby displaying its pixels. This backlight segment turns off again and so that the associated field is never displayed when its pixels are in transition or are not being addressed. Active Addressing is effective because the extra contribution to the square of the pixel voltage, which maintains the on state, is distributed throughout the period of the waveforms.
The row drive waveforms are coupled to the fast-responding STN through a set of switches. There is one switch for each row in the whole display and each switch has two positions. When a row is in the addressed range of rows, the row conductor is connected to one of the row drivers. At other times, the row conductor is connected to ground. It is advantageous to provide more row drivers than there are rows in a subfield which is directly in front of a backlight segment. The extra row drivers are used to address a few extra rows above and below the subfield being addressed. This ensures that an observer who views the display from substantially above or below its centerline in a plane perpendicular to the rows will not see unaddressed pixels illuminated by the backlight segement centered behind the current field.
Grounding the unaddressed rows exposes pixels in them to the column waveforms only. While these waveforms explicitly depend on the pixel values in the addressed range, it turns out that the average of the square of the column voltage over a period is also itself independent of the image. This happens automatically when Active Addressing is used to define just on and off pixel states. When the pixel values are defined in a continuous symmetric range, it is not automatic, but an extra orthogonal waveform can be used in the calculation of the column waveforms. The pixel values of the phantom row are chosen to make the average of the squared column voltage independent of the image. This makes the average of the square of an addressed pixel voltage depend only it""s particular pixel value even though the pixel values are defined on a continuous range. In the present invention, the phantom row also provides a unique off state from which all pixels make their transitions when addressed. This unique RMS column voltage is equal to the voltage required to turn pixels halfway on divided by the square root of 2. In most cases, it can be substantially below the addressing range so that pixels turn on quickly when addressed.
In an illustrative embodiment, a display of N rows is divided into N/M subfields of M rows each, and L extra rows are addressed in the subfields above and below the current one. The period of the orthogonal row waveforms is MT/2N and each field is addressed for 2 periods. The backlight segment associated with the subfield currently being addressed is pulsed during the second period. The M+2L row drivers are each connected to switches at every M+2L rows. The row-signal generator permutes the signals it delivers to the row drivers and the waveforms are delivered in a fixed order to the column-signal generator. For example, waveform Fi can be assigned to connection IC given by ((Kxe2x88x92K0)M+ixe2x88x921)mod(M+2L) where K0 specifies a reference segment where the orthogonal functions Fi are connected to the ith row of the reference field.
An example of the illustrative embodiment is provided by N=240, M=16 and L=4, which are sufficient for a dual-scan VGA or 480p display. The effective multiplex ratio is M+2L+1=25, which is advantageous for obtaining a good horizontal viewing-angle range in planes perpendicular to the columns. It also minimizes the number of row drivers and waveforms. The multiplex ratio could be reduced even more if the emissive surface of the backlight segments can be brought sufficiently close to the LCD. At a frame rate of 60 Hz, MT/2N is about 0.55 msec., which may not be enough time for a fast-responding STN LCD to reach its equilibrium addressed state. Extra time can be provided in this embodiment only to the extent that backlight segments are illuminated for less than 0.55 msec.
In a second embodiment, the rows associated with two or more contiguous backlight segments, plus a few extra rows for parallax reduction, are addressed at one time. While one backlight segment illuminates the rows associated with it, the rows associated with the next one or more backlight segments are pre-addressed. This requires a larger multiplex ratio than the illustrative embodiment, but it allows a liquid crystal with a slower turn-on to be used without reducing the brightness of the display.
The row-signal generator generates (Q+1)M+2L+1 orthogonal signals having period MT/N where Q is a positive integer. Switches are provided at each row to either ground it if it is included in the current field or else to connect it to one of (Q+1)M+2L row drivers. The row-signal generator generates M orthogonal signals for each subfield of the current field. An extra 2L orthogonal waveforms are applied to the first L rows adjacent to the current field in each direction. This provides a correct view in spite of parallax as viewed from directions where a backlight segment illuminates these rows. The final orthogonal signal is not applied to any row, but it is used with phantom pixel values along with the contributions of the (Q+1)M+2L physical rows to the column functions. The phantom pixel values are adjusted to make the average of the column-functions squared independent of the image.
In the second embodiment, (Q+1)M is advantageously also a divisor of N. With this condition, the orthogonal functions applied to the rows i=KM+1, KM+2, . . . (K+1)M can be preassigned to functions FI, where I is given by 1+(ixe2x88x921)mod M+M(K Mod Q+1). The column functions then include the same weighting of these same M functions each time subfield K is a member of the current field. This contribution of M terms due to a subfield is therefore calculated once in each frame and re-used Q times. The subfield pixel values also contribute to the coefficient of the phantom row waveform, which compensates for the continuous range of the pixel values. This contribution can be reused as well, but it is combined with contributions of other subfields in a non-linear way.
Examples of the second embodiment are provided by N=240, M=15, L=4 and Q=1 or M=16, L=4 and Q=2. The effective multiplex ratio is 39 and the turn-on time can be at least 1.04 msec. when the frame rate is 60 Hz. Now, the period of the row waveforms is exactly twice as long as in the illustrative embodiment. Thus, the maximum frequency of the row waveforms is actually lower than in the illustrative embodiment while the turn-on time available is nearly doubled. In the second case, the multiplex ratio is 57 and the turn-on response time can be at least 2.2 msec. when the frame rate is 60 Hz. The maximum frequency of the row waveforms is only a little greater than in the example of the illustrative embodiment that was considered.
In a third embodiment, no extra rows are driven, and the -number of row-driver connections is (Q+1)M, which is advantageously a divisor of N as in the second embodiment. An expanded viewing-angle range in planes perpendicular to the rows is achieved by offsetting the subfields by L rows with respect to the backlight segments. This offset puts the pixels in the last L rows directly in front of the next backlight segment. They are illuminated by segment L only as viewed from the forward edge of the viewing-angle range. An expanded viewing-angle range is achieved by equalizing the transmission of pixels in subfield K during the integration times of backlight segments K and K+1. This equalization is achieved by advancing the integration time so that it ends before the addressing time ends. In the third embodiment, the pixels in all rows are subject to the same addressing time (Q+1)MT/N. Consequently, a liquid-crystal with a turn-on time that is less than the addressing time can be used. The peak light output of the backlight can be increased to compensate for the reduction in the integration time, which is unavoidable because it provides a range of advancement in which equalization can be accomplished. The requirement for increased peak brightness does not necessarily reduce the efficiency of the display. Of course, the efficiency will be reduced when pixels don""t have time to turn on fully and transmit as much light as they could if addressed longer. However, achieving low cost is currently more important than high efficiency for large-area displays for family entertainment.
Equalization is possible over a wide range of turn-on time constants in the example N=240, M=16, Q=2 when the relaxation time constant and the integration time are xcfx84OFF/T=TINT/(MT/N)=0.33. The average pixel transmittance is above 90% for values of xcfx84ON/xcfx84OFF below 0.20. However, equalization breaks down at such values because TADV+TINT rises above MT/N. In this case, the second embodiment could be used because the turn-on time, normally defined as 2.3 xcfx84ON, is sufficiently short. Equalization is possible over a wide range of xcfx84ON/xcfx84OFF, and the average transmission remains above 50% even up to xcfx84ON/xcfx84OFF=0.75. Thus the third embodiment can be used even when the turn-on time is not much shorter than the turn-off time.
The organization and operation of this invention will be understood from a consideration of detailed descriptions of illustrative embodiments, which follow, when taken in conjunction with the accompanying drawings.